A semiconductor light emitting element, such as a light emitting diode or a semiconductor laser, has a general structure wherein an n-type semiconductor and a p-type semiconductor are junctioned together on a semiconductor substrate. Such a light emitting element emits light by employing a radiative recombination of electron-hole pairs, which occurs at a p-n junction. The semiconductor material mainly employed for the light emitting element is a III-V compound semiconductor. This is because the bandgaps (the forbidden bands) of many III-V compound semiconductors are located in the visible light region. Another reason is that, with the development of crystal growth technology in recent years, a quantum well structure, including the p-n junction of the III-V compound semiconductors, can be easily fabricated.
Currently, there are two problems to be solved for the semiconductor light emitting element. The first problem concerns the reduction of power consumption. This problem with the semiconductor light emitting element is requires a high consumption of power, although the device lifetime is comparatively longer than that of lighting equipment that employs a filament or a fluorescent tube, this requires a high consumption of power. The second problem concerns the provision of high luminescence. Since, essentially, these two problems conflict with each other, these problems cannot be simultaneously solved for a conventional light emitting device having a planar p-n junction. That is, for a conventional light emitting element having a planar p-n junction, if the area of the p-n junction is increased in order to increase the luminescence, power consumption and self-absorption would be increased and the effective luminous efficiency would be reduced. And in order to reduce the power consumption, if the injection rate for a current is reduced, the luminescence would be lowered.
As a structure with which these two problems could possibly be solved simultaneously, the focus to date has been on a nanoscale semiconductor structure, such as a semiconductor nanowire structure. Since a semiconductor nanowire has a large ratio of height to diameter, the semiconductor nanowire is characterized in that self-absorption is low, and most of the light that is generated can easily be emitted outside. Therefore, when semiconductor nanowires are employed, the luminous efficiency can be greatly improved. Furthermore, since a semiconductor nanowire has a very small diameter (width), only low-current injection is required for obtaining a radiative recombination of electron-hole pairs. Up to the present, various light emitting elements that employ semiconductor nanowires have been proposed (see, for example, patent document 1 to 6).
In patent document 1, a method is described whereby a p-n junction is formed in the direction of growth (the longitudinal direction) of semiconductor nanowires to fabricate a light emitting element. According to the method in patent literature 1, the VLS method is employed for the fabrication of nanowires.
In patent document 2 and 3, fabrication methods are described for a light emitting element wherein a quantum well structure is provided for nanowires. According to these methods, a p-n junction is formed in the direction of growth of nanowires, and a nano-layer, which is formed of a semiconductor having a smaller bandgap than a p-n junction, is inserted to the p-n junction, so that a quantum well structure is provided in the direction of growth for nanowires. Moreover, according to the method described in patent document 3, the passivation effect on the surface of the semiconductor is employed together with the growth of crystal in the radial direction, so that the luminescence intensity is increased. For the methods in patent document 2 and 3, nanowires are fabricated by employing the VLS method.
In patent document 4 and 5, a light emitting element that has a p-i-n junction in the direction of growth for nanowires is described.
In patent document 6, a method is described whereby a plurality of semiconductor nanowires, which differ in compositions and bandgaps, are formed on a single substrate simultaneously to fabricate a light emitting array composed of a red light emitting element, a green light emitting element, and a blue light emitting element. According to this method, a difference in the diffusion lengths, at which the individual materials travel across an insulating film during the growth of crystal, is used, and a plurality of semiconductor nanowires having different compositions and bandgaps are simultaneously formed on a single substrate.